Improvement of the receiver AGC with the help of a varistor. Scheme, description

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Improvement of the receiver AGC with the help of a varistor. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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By including a varistor in the AGC of the communication receiver, you can increase the stability of the output signal. The varistor together with a fixed resistor form an adjustable voltage divider, which is connected between the RF amplifier and the AGC detector.

This improvement improves the efficiency of the AGC, since the part of the AGC voltage supplied to the controlled RF amplifier varies in proportion to the level of the input signal. Moreover, at low signal levels, this part is negligible, and the amplifier operates at maximum gain, providing the best signal-to-noise ratio. Thus, a varistor AGC has the property of a delayed AGC (or AGC with manual switching), but unlike the latter, it has a continuous control characteristic.

Receiver AGC Improvement with a Varistor

At the maximum AGC voltage (which corresponds to a strong signal), a significant part of it falls on the resistor R1, since the varistor has a low resistance. In this case, the receiver gain is minimal. At the maximum AGC voltage (weak signal), the resistance of the varistor is large, as a result of which a small part of the AGC voltage goes to the resistor R1, and the gain decreases slightly.

The choice of varistor and resistor is made in such a way that at the maximum AGC voltage (highest expected signal) the resistance of the varistor would be half the resistance of the resistor. This condition corresponds to the optimal regulation of the output signal in the range of input signals 1-1000 mV and provides the best signal-to-noise ratio for signals less than 10 μV (due to almost complete shutdown of the AGC circuit).

Specific resistances of the varistor and resistor are selected depending on the signal level at the output of the AGC detector. As a rule, the resistance of the resistor R1 is about 2 MΩ for tube and 100 kΩ for transistor receivers. The time constant of the AGC filter, consisting of a varistor, a resistor and a capacitor C1, changes with the signal level, so the capacitance of the capacitor necessary to filter the modulation components is selected when the varistor resistance is half that of the resistor.

Using an AGC with a varistor in all control stages of a transistor receiver, including an RF amplifier, it is possible to improve sensitivity by 3 dB, reduce receiver input voltage variations by a factor of two for a given change in input level, and increase the absolute value of the output signal by 8-12 dB.

Author: N. Salvati; Publication: N. Bolshakov, rf.atnn.ru

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